Pulls packets at the given RATE, and pushes them out its single output. This
rate is implemented using a token bucket. The capacity of this token bucket
defaults to 20 milliseconds worth of tokens, but can be customized by setting
one of BURST_DURATION or BURST_SIZE.
Keyword arguments are:

RATE

Bandwidth. Token bucket fill rate.

BURST_DURATION

Time. If specified, the capacity of the token bucket is calculated as
rate * burst_duration.

BURST_BYTES

Integer. If specified, the capacity of the token bucket is set to this
value in bytes.